The present invention relates to a novel molten salt monomer and a polymer electrolyte obtained by the polymerization thereof. The present invention further relates to an electrochemical cell, particularly nonaqueous secondary cell and photoelectrochemical cell, comprising such a polymer electrolyte.
The electrolyte to be used in an electrochemical cell such as nonaqueous secondary electrolyte cell and dye-sensitized solar cell is a medium containing ions depending on the purpose capable of transporting the ions between electrodes (ionic conductivity) For example, the performance of a secondary lithium cell representing the nonaqueous secondary cell is governed by the transportation of lithium ion. The performance of dye-sensitized solar cell is governed by the conductivity of iodine ion and iodine trimer ion. These cells normally often comprise a solution system having a high ionic conductivity as an electrolyte but are disadvantageous in that the exhaustion or leakage of the solvent used causes deterioration of durability of the cells in which the electrolyte is incorporated. In the case of secondary lithium cell, a metallic vessel must be used to seal the solution. This adds to the weight of the cell and makes it difficult to have a desired degree of freedom of the shape of cell. In order to overcome these difficulties with such a solution-based electrolyte, various electrolytes have been recently proposed. A so-called gel electrolyte obtained by wetting a polymer matrix with a solution electrolyte exhibits a less drop of ionic conductivity and hence causes a less deterioration of cell properties than the solution-based electrolyte but cannot completely be prevented from the volatilization of the solvent. A polymer electrolyte obtained by dissolving a salt in a polymer such as polyethylene oxide is expected to give solution to the problems with the solution-based electrolyte but leaves something to be desired in ionic conductivity. On the other hand, an imidazolium salt or pyridinium salt the counter anion of which is BF4xe2x88x92, (CF3SO2)2Nxe2x88x92 or the like is a room temperature molten salt which stays liquid at room temperature and has been proposed to be an electrolyte for lithium ion cell. However, such an electrolyte shows a mechanical strength and an ionic conductivity which vary in opposite manner. Thus, when the mechanical strength of the electrolyte is raised by increasing the viscosity of the molten salt itself or incorporating a polymer in the molten salt, there shows a drop of ionic conductivity of the electrolyte. Further, such an electrolyte has a great dependence of ionic conductivity on temperature and thus shows an insufficient ionic conductivity particularly at low temperatures.
By the way, photovoltaic power generation in which light energy is converted to electric energy has been practically realized or studied in the form of single crystal silicon solar cell, polycrystalline silicon solar cell, amorphous silicon solar cell and compound type solar cell comprising cadmium telluride or indium selenide. In order to find wide application of these solar cells, it is necessary to overcome problems involving the production cost, availability of raw materials and length of energy payback time. On the other hand, many proposals have heretofore been made for solar cell comprising an organic material aiming at the increase of working surface area or reduction of cost. However, these proposals are disadvantageous in that they exhibit a low conversion efficiency and a poor durability.
Under these circumstances, xe2x80x9cNaturexe2x80x9d, vol. 353, pp. 737-740, 1991, and U.S. Pat. No. 4,927,721 disclosed techniques concerning a photoelectric element comprising an oxide semiconductor sensitized with a dye (hereinafter simply referred to as xe2x80x9cdye-sensitized photoelectric elementxe2x80x9d) and a photoelectrochemical cell comprising such a dye-sensitized photoelectric element. This cell comprises a photoelectric element which acts as a negative electrode, a charge-transferring layer and a counter electrode. The photoelectric element comprises an electrically-conductive support and a photosensitive layer. The photosensitive layer comprises a semiconductor having a dye adsorbed to the surface thereof. The charge-transferring layer comprises a redox material which acts to transport charge between the negative electrode and the counter electrode (positive electrode). The photoelectrochemical cell proposed in the above-cited patent comprises as a charge-transferring layer an aqueous solution of a salt such as potassium iodide as an electrolyte (electrolytic solution). This system is advantageous in that it is realized at a low cost and can attain a relatively high energy conversion efficiency (photoelectric conversion efficiency) but is disadvantageous in that when used over an extended period of time, the evaporation or exhaustion of the electrolytic solution causes a remarkable deterioration of photoelectric conversion efficiency or makes it impossible for the cell to operate properly.
Referring to these problems, as a countermeasure against the exhaustion of electrolytic solution there is disclosed a method involving the use of an imidazolium salt, which is a low melting compound, as an electrolyte in WO 95/18456. The employment of this method eliminates or lessens the necessity of water or an organic solvent, which has heretofore been used as a solvent for electrolyte, making it possible to improve the durability of the electrolyte. However, this electrolyte leaves something to be desired in durability. This method is also disadvantageous in that when the concentration of the imidazolium salt increases, the resulting photoelectric conversion efficiency is deteriorated. Another method has been proposed involving the use of a triazolium salt as an electrolyte. However, this method has the same problems as imidazolium salt.
An object of the invention is to provide an electrolyte for an electrochemical cell having excellent durability and ionic conductivity and a photochemical cell having excellent durability and photoelectric properties. Another object of the invention is to provide a novel molten salt monomer for use in such an electrolyte.
Other objects and effects of the present invention will become more apparent from the following description.
The foregoing objects of the invention have been achieved by providing the following polymerizable molten salt monomers, electrolyte compositions, electrochemical cell, photoelectrochemical cell and nonaqueous secondary cell.
(1) A polymerizable molten salt monomer represented by the following general formula (I): 
wherein Q represents a nitrogen-containing aromatic heterocyclic atomic group which can form a cation; Y1 represents a divalent interlocking group or a bonding hand; Y2 represents a substituted or unsubstituted alkyl group; n represents an integer of from 2 to 20; m represents an integer of 2 or more; Xxe2x88x92 represents an anion; plural Y1""s and plural Y2""s may be the same or different, respectively, with the proviso that at least one of Y2""s has a polymerizable substituent group; and a plurality of the compounds of the general formula (I) may be connected to each other at Q or Y2 to form a dimer, trimer or tetramer.
(2) The polymerizable molten salt monomer according to the above-item (1), wherein the general formula (I) is represented by the following general formula (II): 
wherein Y1 represents a divalent interlocking group or a bonding hand; Y2 represents a substituted or unsubstituted alkyl group; R1 represents a substituent; n represents an integer of from 2 to 20; ml represents an integer of from 2 to 5; m2 represents an integer of from 0 to (5xe2x88x92m1); Xxe2x88x92 represents an anion; plural Y1""s and plural Y2""s may be the same or different, respectively, with the proviso that at least one of Y2""s has a polymerizable substituent group; and a plurality of the compounds of the general formula (II) may be connected to each other at R1 or Y2 to form a dimer, trimer or tetramer.
(3) The polymerizable molten salt monomer according to the above item (1), wherein the general formula (I) is represented by the following general formula (III): 
wherein Y1 represents a divalent interlocking group or a bonding hand; Y2 represents a substituted or unsubstituted alkyl group; R1 represents a substituent; n represents an integer of from 2 to 20; m1 represents an integer of from 2 to 6; m2 represents an integer of from 0 to (6xe2x88x92m1); Xxe2x88x92 represents an anion; plural Y1""s and plural Y2""s may be the same or different, respectively, with the proviso that at least one of Y2""s has a polymerizable substituent group; and a plurality of the compounds of the general formula (III) may be connected to each other at R1 or Y2 to form a dimer, trimer or tetramer.
(4) The polymerizable molten salt monomer according to any one of the above items (1) to (3), wherein said polymerizable group is an ethylenically unsaturated group.
(5) The polymerizable molten salt monomer according to any one of the above items (1) to (4), wherein said polymerizable group contains a group selected from the group consisting of an acryloyl group, a methacryloyl group and a styryl group.
(6) The polymerizable molten salt monomer according to any one of the above items (1) to (5), wherein said polymerizable group is represented by the following general formula (IV): 
wherein R2 represents a hydrogen atom or an alkyl group; xe2x80x94Y3xe2x80x94 represents xe2x80x94Oxe2x80x94, xe2x80x94N(R3)xe2x80x94 or a single bond; and R3 represents a hydrogen atom or an alkyl group.
(7) The polymerizable molten salt monomer according to any one of the above items (1) to (6), wherein Xxe2x88x92 in the general formulae (I) to (III) is a halogen anion, an amide anion or a fluoride anion containing at least one element selected from the group consisting of boron (B), phosphorus (P) and sulfur (S).
(8) The polymerizable molten salt monomer according to any one of the above items (1) to (7), wherein Xxe2x88x92 in the general formulae (I), (II) and (III) is an iodine anion.
(9) An electrolyte composition containing a polymer compound obtained by polymerizing a polymerizable molten salt monomer according to any one of the above items (1) to (8).
(10) The electrolyte composition according to the above item (9), further comprising iodine.
(11) The electrolyte composition according to the above item (9) or (10), further comprising a lithium salt.
(12) An electrochemical cell containing an electrolyte composition according to any one of the above items (9) to (11).
(13) A photoelectrochemical cell comprising:
a charge-transferring layer containing an electrolyte composition according to any one of the above items (9) to (11);
a photosensitive layer containing a semiconductor sensitized with a dye; and
a counter electrode.
(14) A nonaqueous secondary cell containing an electrolyte composition according to any one of the above items (9) to (11). The nonaqueous secondary cell preferably comprises an electrochemical cell according to the above item (12), which comprises a positive electrode, a negative electrode and a lithium salt.